The invention concerns an ion mobility spectrometer according to the preamble of claim 1.
There is an example of a prior-art ion mobility spectrometer of the cited kind in DE 15 91 345.9 or U.S. Pat. No. 4,390,784. Such an ion mobility spectrometer can detect even small portions of substances to be analysed in a carrier gas and usually consists of an ionisation chamber in which the molecules of the substance to be analysed and the carrier gases are ionised. Connected to this ionisation chamber is a drift chamber. The ions enter it via an electrically switchable ion gate. Between the ion gate and an ion collector on the opposite end of the drift chamber is an axially aligned electrical field. The ions formed in the ionisation chamber move along the drift path in the field toward the ion collector. The ion gate is switched so that it lets a swarm of the ion mixture which is to be analysed into the drift chamber. While drifting toward the ion collector, this swarm is divided into partial swarms that are characteristic for the components of the mixture. The ions are neutralized on the conductor surface of the ion collector, and their charge is released. This is also described as ion discharging on a potential-conducting surface. The partial swarms contact the ion collector at different times and are detected by means of signal electronics. The received signal allows conclusions to be drawn about the analysed mixture.
U.S. Pat. No. 4,777,363 discloses the ion gate as an arrangement of parallel wires that run perpendicular to the drift axis. All even wires are electrically connected to each other, and all uneven wires are electrically connected to each other. Between the two wires groups obtained in this manner, a voltage is applied to block the ion gate which is known in the literature as a Bradburry-Nielson arrangement. If the two wire groups are at the same potential, the gate is opened to the ions. If a voltage is applied between them, the ions are led to the grid wires where they release their charges. To reduce the influence of mirror charges, this device also uses an aperture grid in front of the ion collector; however, ions are lost by being discharged there.
U.S. Pat. No. 4,390,784 discloses an ion collector at the end of the drift path that is perpendicular to the drift axis and whose ion-discharging surface extends nearly over the entire cross-section of the drift chamber.
A purge gas such as nitrogen or air usually flows through the drift chamber. U.S. Pat. No. 4,390,784 has a feed for the purge gas at the downstream end of the drift chamber and a drain at the upstream end of the drift chamber. Since the purge gas has to be guided past the collector, it cannot cover the entire cross-section of the drift chamber. In the prior-art devices, however, only a small percent of the collector surface is lost from the gas flow since the gas flow is typically xe2x85x9 or ⅙ inch, and the drift chamber cross-section is 1-2 cm (U.S. Pat. No. 4,390,784).
If, however, one wishes to miniaturize the cross-section of an ion mobility spectrometer, the surface loss due to the gas flow is a problem. A similar problem arises with the ion gate in the miniaturization of ion mobility spectrometers. A conventional Bradburry-Nielson wire grid is difficult to miniaturize since a sufficient number of grid wires cannot be placed in the miniaturized cross-section with the given wire thickness.
It is therefore the task of the present invention to create an ion mobility spectrometer that is suitable for miniaturization.
The cited problem is solved with the features of patent claim 1. Advantageous developments are characterized in the subcdaims. According to the invention, the potential-conducting surface of the ion gate or ion collector forms an angle with the drift chamber axis that is much less than 90xc2x0. This makes it possible to provide the required openings in the faces of the drift chamber to feed and remove the purge gas without limiting the size of the potential-conducting surface. This allows the cross-section of the ion mobility spectrometer to be miniaturized. In addition, the above-cited aperture grid can be dispensed with since the effect of influence charges of the ion swarm is much less due to the small cross-sectional surface of the ion collector in the drift axis direction.
The ion-discharging surface of the collector can of course be composed of several sections that are electrically connected to each other or are connected directly to the signal detecting electronics. Since the ion-discharging surfaces form an angle less than 90xc2x0 with the drift chamber axis, there remains space for an opening for a gas flow in a central, inner radial area of the drift chamber. The collector surface therefore encloses the gas flow.
Since the surface of the ion gate forms an angle much less than 90xc2x0 with the drift chamber axis, the gas can flow unhindered through it like at the ion collector.
It is in particular possible to place the potential-conducting surface of the ion gate and the ion collector inside on the drift chamber wall. In certain cases, an insulating layer must be between the potential-conducting surface and the inner wall of the drift chamber.
In a preferred embodiment, the size of the potential-conducting surface of the ion collector is about the same of the opening perpendicular to the drift direction through which the drifting purge gas is guided to allow the collector to collect all the ions at the end of the drift path.